Electronic circuit

ABSTRACT

An electronic circuit for performing logic operations is provided. The electronic circuit comprises a logic gate having at least two binary inputs adapted to receive corresponding input binary digits; an output for outputting an output signal; signal transmission means between said input and said output; a logic circuit coupled to said transmission means and having an input capacitance, and capacitance decoupling means between said logic circuit and said transmission means for decoupling the input capacitance of said logic circuit from said transmission means.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser. No. 60/657,391, filed Mar. 2, 2005 the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to electronic circuits, and in particular to electronic circuits for performing logic operations.

Logic circuits have a wide variety of uses and may be configured to perform arithmetical operations. A basic full adder circuit can be described by the following two equations: Sum=A⊕B⊕C_(in)   (Eq. 1) Cout=AB+BC_(in)+AC_(in)   (Eq. 2)

A full adder cell or block can be illustrated in the form shown in FIG. 1. The full adder cell 1 has first and second inputs 3, 5 for receiving the numbers (A, B) to be summed, a carry input 7, a carry output 9 and a sum output 11. Typically each input and output is a single bit. In order to perform higher order addition, a number of full adders are connected together in a series with the carry output of one adder being connected to the carry input of another adder. An example of a four-bit adder circuit is shown in FIG. 2 and comprises four full adder cells 2, 4, 6 and 8. The main concerns in designing a full adder circuit are the speed and area of the circuit. In the four-bit full adder shown in FIG. 2, the maximum propagation delay can be described by: 4bit_delay=t_(H)+4×t_(carry)+t_(Sum)   (Eq. 3) where t_(H) describes the delay through the circuit that implements (Eq. 4) logic: H={overscore (A)}B+A{overscore (B)}=A⊕B  (Eq.4) Using (Eq.4), (Eq.1) and (Eq.2) are rewritten as: Sum=H⊕C_(in)   (Eq. 5) Cout={overscore (H)}A+HC_(in)   (Eq. 6) t_(carry) describes the delay from C_(in) to C_(out) of a full adder cell, and t_(Sum) describes the delay through the circuit that implements Eq. 5. The reason that the H-equation logic is added is to minimize the number of input lines that effect the propagation of C_(in) to C_(out).

FIG. 3 shows a typical standard cell implementation of a full adder which implements equations 4, 5 and 6. The adder circuit comprises a first (or input) XOR gate 15 for determining the value of H (equation 3), an inverter 17 at the output of the XOR gate 15 for deriving the value {overscore (H)} for use in equation 6, a first transmission gate 19, a carry propagate circuit 21 (transmission means) between the carry input and carry output 7, 9, and a summing circuit 31 comprising a second XOR gate for implementing equation 5. The carry propagate circuit 21 comprises a second transmission gate 23 and a buffer 25 comprising a pair of serially arranged inverters 27, 29.

The first input 20 of the first transmission gate 19 is connected to the first input 3 of the adder circuit and the other complementary inputs 22, 24 (NMOS gate and PMOS gate) are connected to receive the values {overscore (H)} and H respectively. The output 26 of the first transmission gate 19 is connected to the carry propagate circuit 21 and implements the first term of equation 6. The first input 28 of the second transmission gate 23 is connected to the C input 7 of the carry propagate circuit and the complementary inputs 30, 32 are connected to receive the values of H and {overscore (H)}, respectively. The output 34 of the second transmission gate outputs the value HC_(IN) which is the second term of equation 6.

One of the inputs 36 of the second (or summing) XOR gate 31 is connected to the output of the first XOR gate 15 for receiving the value H and the second input 38 of the second XOR gate is connected to the input 7 of the carry propagate circuit 21 for receiving the value C_(IN).

The components of the full adder circuit shown in FIG. 3 that are responsible for the propagation delay of equation 3 are as follows. t_(H) is the delay through the input XOR gate 15, t_(SUM) is the delay through the output XOR gate 31, and t_(CARRY) is the delay from C_(IN) to C_(OUT) (for bit 0 this might be from the first input 3 (A) to C_(OUT)). It is apparent from equation 3 that the propagation delay from C_(IN) to C_(OUT) in a multiple cell adder is more heavily weighted than other parts of the equation, and this delay plays a pivotal role in determining the total delay through the adder chain.

In a typical standard cell implementation, when H=1, the C_(IN) signal is loaded by the gate capacitance of the second XOR gate 31, which is equal to, at the very least, the input capacitance of an inverter comprising an N-type and P-type gate capacitance; the N-type and P-type gate capacitance of the second transmission gate 23, the two drain to gate capacitances of the first transmission gate 19 and the input capacitance of the buffer cell 25. In addition, there is a propagation delay through the output buffer cell, which comprises two inverters 27, 29 in series.

It would be desirable to reduce the propagation delay in the carry propagate circuit.

SUMMARY OF THE INVENTION

The present invention in one embodiment involves the provision of an electronic circuit comprising: a logic gate having at least two binary inputs adapted to receive corresponding input binary digits; an output for outputting an output signal; signal transmission means between said input and said output; a logic circuit coupled to said transmission means and having an input capacitance, and capacitance decoupling means between said logic circuit and said transmission means for decoupling the input capacitance of said logic circuit from said transmission means.

According to another aspect of the present invention, there is provided an electronic circuit comprising first and second signal generating means, each having an input and an output, circuit means between the output of said first signal generating means and the input of said second signal generating means, and a circuit, for example a logic circuit coupled to the input of said first signal generating means.

According to yet another aspect of the present invention, there is provided an electronic circuit comprising first and second inputs and first and second outputs, first transmission means between said first input and said first output and second transmission means between said second input and said second output, a circuit having first and second inputs, wherein the first input is coupled to the first transmission means and the second input is coupled to the second transmission means.

According to a further aspect of the present invention, there is provided an electronic circuit comprising first and second transmission means having a respective input and a respective output and a differential buffer having a first input coupled to the output of said first transmission means and a second input coupled to the output of said second transmission means.

According to yet a further aspect of the present invention, there is provided an adder circuit comprising first and second inputs for receiving signals to be added together, circuit means for determining the sum of said signals and first and second carry propagate circuits each having an input and an output, and wherein said first carry propagate circuit is adapted to determine a carry-out signal from a carry-in signal and the second carry propagate circuit is adapted to determine a carry-out signal from an inverted carry-in signal.

According to yet a further aspect of the present invention, there is provided an electronic circuit comprising an input circuit having at least one input for receiving a signal, a transmission means having an input and output, the input circuit being connected to said transmission means, a further circuit coupled to the transmission means, and capacitive reduction or decoupling means connected between said transmission means and said further circuit means for reducing the capacitive load on the input of said transmission means.

BRIEF DESCRIPTION OF DRAWINGS

Examples of embodiments of the present invention will now be described with reference to the drawings, in which:

FIG. 1 shows a schematic block diagram of a full adder circuit;

FIG. 2 shows a block diagram of a full adder comprising multiple cells;

FIG. 3 shows an example of a full adder circuit;

FIG. 4 shows an example of a full adder circuit according to an embodiment of the present invention;

FIG. 5 shows an example of a full adder circuit according to another embodiment of the present invention;

FIG. 6 shows an example of a full adder circuit according to another embodiment of the present invention;

FIG. 7 shows an example of a full adder circuit according to another embodiment of the present invention;

FIG. 8 shows an example of an electronic circuit according to another embodiment of the present invention; and

FIG. 9 shows an example of an electronic circuit according to another embodiment of the present invention.

It should be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. It should be understood, of course, that the invention is not necessarily limited to the particular embodiments illustrated herein. Like numbers utilized throughout the various Figures designate like or similar parts.

DETAILED DESCRIPTION

Before describing the present invention, it will be understood that variations of the present invention may is not limited to the specific examples described herein.

FIG. 4 shows an example of a full adder circuit according to a first embodiment of the present invention.

The circuit 101 comprises first and second inputs 103, 105 for receiving bits to be summed and each being connected to the respective inputs of an input XOR gate 115 for generating a value “H”. The first inverter 117 is connected to the output of the input XOR gate 115 for generating a value {overscore (H)}, and the values {overscore (H)} and H are applied to the complementary inputs of a first transmission gate 119. The other input terminal 120 of the first transmission gate 119 is arranged to receive the inverse of the signal received at the first input 103. Although {overscore (A)} may be generated by a separate inverter 110, the value {overscore (A)} is also generated by the XOR gate 115 and therefore a separate inverter is not required.

The adder 101 has a carry propagate circuit 121 having an input 107 for receiving a carry bit, C_(IN), and an output 109 for outputting a carry bit, C_(OUT). The propagate circuit 121 comprises a first inverter 151, a transmission gate 123 and a second inverter 153. The output 126 of the first transmission gate 119 is connected to the carry propagate circuit between the transmission gate 123 and the second inverter 153.

The adder 101 includes a second XOR gate 131 for generating a sum from the values of H and C_(IN) (equation 5) and a buffer circuit 155 connected between the carry propagate circuit 121 and the summing XOR gate 131.

The buffer circuit 155 has a first input 157 connected to the input of the first inverter 151 and a second input 159 connected to the output of the first inverter. The buffer circuit 155 comprises first and second FET's 161, 163 and third and a fourth FET's 165, 167. The first and third FET's 161, 165 are both NMOS-type FETS whereas the second and fourth FET's 163, 167 are both PMOS-type FETS. The sources of the first and third FET's are connected to ground and their drains are connected to the sources of the second and fourth FET's, respectively. The drains of the second and fourth FET's are connected to a voltage rail 169. The gate of the first FET 161 is connected to the input of the first inverter 151 of the carry propagate circuit and the gate of the third FET 165 is connected to the output of the first inverter 151. The gate of the second FET 163 is cross-coupled to the drain of the second FET 165 and the gate of the fourth FET 167 is cross-coupled to the drain of the first FET 161. The drains of the first and third FET's 161, 165 provide inverted and non-inverted buffered carry-in signals C_(BUF) and C_(BUF), respectively and are used as inputs to the XOR gate 131, which will be described in more detail below.

A typical XOR gate comprises a pair of AND gates each having an inverter on one of their inputs and an OR gate connected to their outputs. As an inverter and the input to an AND gate typically comprise two FET's each when implemented in CMOS, each input to an XOR gate is effectively coupled to four gate capacitances.

The purpose of the buffer circuit 155 is to effectively reduce the capacitance at the input of the carry propagate circuit. The buffer circuit effectively decouples the input capacitance of the summing circuit 131 from the input 107 of the carry propagate circuit.

In the embodiment shown in FIG. 4, the input capacitance of the carry propagate circuit essentially corresponds to the gate capacitance of the first NMOS FET 161 and the two-gate capacitances of the FET's of the first inverter 151. This is a substantial reduction in the input capacitance of the circuit shown in FIG. 3. It is to be noted that the gate capacitance of the first FET 161 can be reduced by using a relatively small FET. Also, as the FET can be implemented in NMOS, the gate capacitance can be significantly reduced over a PMOS FET, in which the gate area (and therefore capacitance) is typically 3 times larger. The “input” capacitance of the carry propagate circuit at the output of the first inverter 151 is provided by the gate capacitance of the third NMOS FET 165 of the buffer circuit, the gate capacitances of the NMOS and PMOS FET's of the transmission gate 123, the gate to drain capacitances of the transmission gate 119 and the input capacitance of the second inverter 153.

Partially coupling the summing XOR circuit 131 at the input of the first inverter 151 causes the capacitive loading of the carry propagate circuit to be shared more evenly between the inverters, each of which drives an inverted input signal from its output. The presence of two inverters ensures that the parity of the carry propagate signal is maintained between the input and output of the carry propagate circuit 121.

The differential buffer circuit 155 operates as follows. When the input of the carry propagate circuit is high (i.e. 1), the gate of the first FET 161 is high connecting the drain of the first FET to ground, and generating {overscore (C)}_(BUFF). At the same time, the output of the first inverter and therefore the gate of the third FET 165 is low, turning the third FET off. The gate of the fourth FET 167 (PMOS) is pulled low, turning on the FET and connecting its source to the voltage rail 169 so that C_(BUF) is high. This signal drives the gate of the second FET 163 (PMOS) high, thereby turning the second FET 163 off, to ensure that its source is pulled low by the first FET 161. In this way, the differential buffer circuit 155 generates inverted and non-inverted signals in which the voltage of the high signal corresponds to the voltage on the voltage rail 169 and the low signal essentially corresponds to ground.

When the carry input signal is low (i.e. 0), the first FET 161 is switched off, the third FET 165 is switched on, thereby connecting the gate of the second FET 163 to ground, and turning on the second FET 163, so that {overscore (C)}_(BUF) is high (i.e. 1) and is equal to the voltage of the voltage rail 169. This signal drives the gate of the fourth FET 167 low turning off the fourth FET 167, so that C_(BUF) is low (i.e. 0) and corresponds to ground.

In this embodiment, the XOR gate 131 comprises first and second AND gates 171, 173, an inverter 175 at the input of the second AND gate 173 and an OR gate 177 to which the outputs of the AND gates are connected. Although a conventional XOR gate would normally have an inverter at the first input 172 of the first AND gate 171, the differential buffer circuit 155 allows this inverter to be omitted as the circuit provides the inverted carry-in signal C_(BUF). It is to be noted that {overscore (H)} is also available at the output of the inverter 117 connected to the output of the input XOR gate 115, and this signal could be provided at the input of the second AND gate 173, allowing inverter 175 to be eliminated.

The presence of the first inverter 151 in the propagate carry circuit changes the polarity of the carry input signal which is available to the succeeding portion of the circuit (including the transmission gate 123) which determines C_(OUT) using equation 6. As the polarity of this signal is reversed by the second inverter 153, the carry signal at the input of the second inverter 153 is given by the equation: {overscore (C)}={overscore (C)}_(IN)H+{overscore (AH)} and C_(OUT) at the output of the second inverter is given by the equation: C_(OUT)={double overscore ({overscore (C)}H+{double overscore (AH)}. Therefore, the value H and {overscore (C)} are applied to the transmission gate 123 and {overscore (AH)} are applied to the first transmission gate 119 to generate {overscore (C)} at the input of the second inverter 153, and the second inverter then generates C_(OUT).

Thus, in comparison to the circuit of FIG. 3, in the embodiment of FIG. 4, an inverter replaces the output buffer and a second inverter is placed at the input to maintain signal polarity. To decrease the amount of load on the C_(IN) to C_(OUT) signal path the differential buffer cell 155 is inserted between the carry input 107 and the sum XOR gate 131. This differential buffer 155 connects to the input C_(IN) using the gate input of a minimum size NMOS gate, and attaches to the inverted version of C_(IN) using a similar minimum size NMOS gate.

By using the differential buffer, the overall capacitance on the carry line can be reduced, and the load per inverter in the chain can be shared more evenly. By reducing the load on C_(IN) and sharing the load more evenly, a higher propagation speed can be achieved.

As mentioned above, although FIG. 4 shows an inverter 110 to generate {overscore (A)}, the inverter is not in practice required as {overscore (A)} is generated within the XOR gate that generates H. The differential buffer 155 creates both {overscore (C)}_(BUF) and C_(BUFF), which means that an inverter (or two) can be removed from the sum XOR gate. The cost of this new implementation over the standard circuit is at most two transistors for an unbuffered (slower—more load on C_(IN))-adder.

The embodiment of FIG. 4 is suitable for voltage scaling applications because all nodes including the transmission gates realize the full voltage swing. If voltage scaling is not important to the application, or if the application supports multiple on-chip voltage levels so that this circuit can be kept in a range where it works efficiently, then one or more of the transmission gates may be replaced by a pass transistor, for example an NMOS pass transistor. An example of this implementation is shown in FIG. 5. The embodiment of FIG. 5 is similar to that shown in FIG. 4 and like parts are designated by the same reference numerals (However, the XOR gates have been omitted for simplicity, although they would, of course, be present in the actual implementation). The main difference between the circuit of FIG. 5 and that of FIG. 4 is that in FIG. 5, the transmission gates 119, 123 have been replaced by pass gates 150, 160. Replacing the transmission gate 123 in the carry propagate circuit 121 with a pass gate reduces the delay through the circuit by removing a PMOS drain-source (DS) capacitance, and a PMOS gate capacitance associated with the PMOS FET of the transmission gate. At the input of the second inverter 153, the voltage level will be reduced to V_(DD)-V_(THN). This will only be an issue if the voltage rail is too low and the input voltage to the inverter does not reach a high enough level to cause it to switch. As long as a sufficient voltage margin is maintained, this circuit should operate quickly and efficiently. In a variation of the embodiment of FIG. 5, either one of the first and second pass gates 150, 160 can be replaced by a transmission gate.

In another embodiment of the propagate circuit, the carry-out output inverter (e.g. inverter 153) can be omitted thereby removing the propagation delay caused by the output inverter and further increasing the speed of propagation through the circuit. In this implementation, as each carry propagate circuit contains only one inverter, the polarity of the carry signal will be inverted once per circuit and therefore returned to the original polarity after propagating through two adjacent adder circuits. An example of this implementation is shown in FIG. 6.

Referring to FIG. 6, an adder circuit comprises two full adders 201, 203, (EVEN and ODD) each of which is similar to that shown in FIG. 5 and like parts are designated by the same reference numerals. The difference between the first full adder circuit 201 and that shown in FIG. 5, is that the output inverter 153 has been omitted. In this embodiment, since the carry-out (C_(OUT)) determining circuit of the EVEN adder 201 receives {overscore (C_(IN))} after the inverter 151, the input signals to the pass gate 150 are {overscore (A)} and {overscore (H)}. Although the circuit shows the inclusion of an inverter 110 to generate {overscore (A)}, since {overscore (A)} is available from the input XOR gate (shown in FIG. 4, not in FIG. 6), the additional inverter is not required in practice.

The input to the second full adder 203 is {overscore (C_(OUT))} which is subsequently inverted by the inverter 151 of the second adder 203. Therefore, the carry signal which is input to the carry-out determining circuit of the second adder 203 has the original polarity of C_(IN) applied to the input of the first adder 201 and therefore the input signals to the pass gate 150 of the second circuit is A and {overscore (H)} (rather than {double overscore (A)} and {double overscore (H)}, for the first circuit).

A differential buffer cell 155 is connected across the inverter 151 of the carry propagate circuit of each full adder 201, 203 to reduce the capacitive loading at the input of the carry propagate circuit of each adder 201, 203, caused by the input capacitance of each summing XOR gate (shown in FIG. 4, not in FIG. 6). Each differential buffer circuit generates both an inverted and a non-inverted carry-in signal (i.e. C0 _(BUF), C0 _(BUF) for the first adder and C1 _(BUF) and C1 _(BUF) for the second adder 203) which can be applied to the inputs of the XOR gate associated with each adder.

By removing the output inverter and dividing the circuit into odd and even bits, a gate delay is saved. The actual implementation will not save the complete gate delay due to the inverter because the single inverter that remains is required to drive a modestly larger load as there is no additional inverter with which to share the load. However, the reduction in delay caused by the omission of the output inverter is significant. As an optional but preferable feature, the last full adder in the chain is arranged such that C_(OUT) drives either a small load or C_(OUT) is buffered.

In another embodiment of the present invention, the carry propagate circuit is split into two carry propagate lines so that the capacitive loading on each line can be further reduced. Each line may include a single inverter and the input of one line may be an inverted carry signal and the input of the other line may be a non-inverted carry signal. A differential buffer may be connected between the lines and the summing XOR gate to decouple the input of the carry propagate circuit from the capacitive loading of the XOR gate. One of the inputs to the differential buffer circuit may be connected to the input of one of the carry propagate lines and the other input of the differential buffer may be connected to the input of the other carry propagate line. The single inverter on each line may be implemented by a differential buffer circuit which can be arranged to reduce the capacitive loading on each line in comparison to a conventional double inverter-type buffer.

An example of an embodiment of a carry propagate circuit having two propagate lines is shown in FIG. 7.

Referring to FIG. 7, a full adder circuit 301 comprises first and second carry propagate lines 351, 353 each having an input 355, 357 and an output 359, 361. Each line includes a circuit for determining the value of the carry-out signal. The first line 351 is connected to pass gate 350 for generating {overscore (AH)} and also includes pass gate 360 for generating C_(IN)H. The second line 352 353 is connected to pass gate 352 for generating {overscore (AH)} or {overscore (AH)}and also includes pass gate 362 for generating {overscore (C_(IN))}H or {overscore (C_(IN)H)}.

A differential buffer circuit 365 is connected between the carry inputs 355, 357 and a summing XOR gate (not shown) and has a first input 367 connected to the first carry input 355 and a second input 369 connected to the second carry input 357. Advantageously, connecting a single input of the differential buffer to each carry propagate line effectively reduces the capacitive loading of the differential buffer on the carry propagate lines in comparison with the embodiment of FIG. 4, for example, since each line is connected to only one gate capacitance of an NMOS FET rather than two NMOS FET gate capacitances.

A second differential buffer circuit 381 is connected to each carry propagate line and may be connected in a manner which preserves the polarity of the carry signal in the upper and lower outputs 359, 361. The second differential buffer circuit is similar to that between the carry input lines and the summing XOR gate and fuictions in a similar manner, as described above. In particular, the differential buffer comprises first and second FET's 383, 385 in which the drain of the first FET is connected to the source of the second FET 385. The differential buffer circuit also comprises third and fourth FET's 387, 389 in which the drain of the third FET 387 is connected to the source of the fourth FET 389. The gates of the first and third FET's (which are preferably NMOS FET's) are connected to the inverse carry and non-inverted carry lines 357, 355, respectively and the gates of the second and fourth FET's (which are preferably PMOS FETS) are cross-coupled to the drains of the third and first FET's, respectively.

The reduction in capacitive loading of each carry propagate line provided by this circuit significantly increases the speed of the circuit, and the circuit exhibits good noise immunity due to its differential nature.

In an alternative embodiment, the differential buffer at the output of the circuit of FIG. 7 may be replaced by one or more inverters in each line. At least one of these inverters is preferably connected differentially, cross-coupled, between the carry propagate lines to preserve timing between the carry propagate lines and reduce the risk of a race condition between Cin and {overscore (Cin)}. However, one of the benefits of a differential buffer at the output of the propagate lines is the reduced capacitance since each line is loaded by the gate capacitance of an NMOS FET which is smaller than the input capacitance of a buffer which is typically provided by the gate capacitances of both an NMOS and PMOS FET.

In another embodiment, one or more pass gates of the embodiment of FIG. 7 may be replaced by a transmission gate.

FIGS. 8 and 9 each show another embodiment of the present invention. The embodiments of FIGS. 8 and 9 are similar to that shown in FIG. 4, and like parts are designated by the same reference numerals. The main difference between the embodiment shown in FIG. 8 and that shown in FIG. 4 is that in FIG. 8, the differential buffer connected to the input 107 of the carry propagate line has been replaced by an alternative embodiment of a capacitive reduction or decoupling means to reduce the capacitive loading on the carry propagate input resulting from the input capacitance of a logic circuit (e.g. summing XOR gate) to which it is connected. In this embodiment, the capacitive decoupling means comprises a transistor 161, for example an FET, e.g. NMOS or PMOS having a source, drain and gate, in which the gate is connected to the carry propagate input 107. The capacitive decoupling or reduction circuit further comprises a resistive load 164 coupled between the transistor 161 and the voltage rail 169. If the transistor 161 comprises an FET, either the drain or source may be connected to the resistive load 164. The other of the drain and the source of an FET implemented transistor 161 is connected to ground. In this embodiment, the capacitive decoupling circuit outputs {overscore (C_(BUF) )} at the junction between the transistor 161 and the resistive load 164 for input to a logic circuit. In another embodiment, the control input (e.g. gate) of the transistor 161 can be connected to the output of the first inverter 151, in which case the capacitive decoupling circuit would output C_(BUF) to be input to a logic circuit connected thereto.

The resistor or resistive load 164 can be implemented by any suitable means such as n-type or a p-type transistor. In another embodiment, the resistive load may comprise a device that can be implemented as a resistor such as a bipolar device, a length of poly (e.g. poly silicon) or any other device.

Referring to the embodiment shown in FIG. 9, the main difference between this embodiment and that shown in FIG. 8 is that the first inverter 151 has been removed from the carry propagate circuit, in which the case output of the carry propagate circuit is {overscore (C)}_(OUT).

Any of the full adder circuits disclosed herein may be used to implement a full adder of any size.

In other embodiments, the various transistors used to implement the full adder circuit (or other circuit), may be of any other suitable type, for example bipolar. NMOS and PMOS FETS disclosed herein are illustrative examples only and any FET of either type disclosed herein may be replaced by any other type, as desired. Furthermore, any references herein to a source of an FET may be replaced by a reference to a drain, and vice versa.

The principles of reducing the capacitive loading and load sharing provided by various aspects of the present invention may be applied to other circuits for increasing the speed of transmitting a (digital) signal.

Thus, there has been shown and described several embodiments of a novel invention. As is evident from the foregoing description, certain aspects of the present invention are not limited by the particular details of the examples illustrated herein, and it is therefore contemplated that other modifications and applications, or equivalents thereof, will occur to those skilled in the art. The terms “having” and “including” and similar terms as used in the foregoing specification are used in the sense of “optional” or “may include” and not as “required”. Many changes, modifications, variations and other uses and applications of the present construction will, however, become apparent to those skilled in the art after considering the specification and the accompanying drawings. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow. 

1. An electronic circuit comprising: a logic gate having at least two binary inputs adapted to receive corresponding input binary digits; an output for outputting an output signal; signal transmission means between said input and said output; a logic circuit coupled to said transmission means and having an input capacitance; and capacitance decoupling means between said logic circuit and said transmission means for decoupling the input capacitance of said logic circuit from said transmission means.
 2. An electronic circuit as claimed in claim 1, wherein said decoupling means comprises a field effect transistor (FET), having a gate, a source and a drain.
 3. An electronic circuit as claimed in claim 2, wherein said gate is connected to said transmission means and one of said source and said drain is coupled to said logic circuit.
 4. An electronic circuit as claimed in claim 3, wherein said FET comprises an NMOS or PMOS type FET.
 5. An electronic circuit as claimed in claim 1, further comprising signal regenerating means for regenerating the signal transmitted from said decoupling means to said logic circuit.
 6. An electronic circuit as claimed in claim 1, wherein the decoupling means has an input capacitance that is smaller than the input capacitance of said logic circuit.
 7. An electronic circuit as claimed in claim 1, wherein said decoupling means comprises means for generating an inverted signal and a non-inverted signal from a signal on said transmission means.
 8. An electronic circuit as claimed in claim 1, wherein said transmission means comprises a logic circuit having an input coupled to the input of said transmission means and an output coupled to the output of said transmission means and said decoupling means is coupled to at least one of the input and the output of the logic circuit in said transmission means.
 9. An electronic circuit as claimed in claim 8, wherein said decoupling means is coupled to both the input and the output of the logic circuit of said transmission means.
 10. An electronic circuit as claimed in claim 9, further comprising means for regenerating the signal at at least one of the input to said logic circuit of said transmission means and the output from the logic circuit of said transmission means.
 11. An electronic circuit as claimed in claim 9, wherein the logic circuit of said transmission means comprises an inverter.
 12. An electronic circuit as claimed in claim 10, wherein said logic circuit in said transmission means comprises an inverter and said signal regenerating means is adapted to regenerate both an inverted signal and a non-inverted signal from the signals at the input and the output of said logic circuit of said transmission means.
 13. An electronic circuit as claimed in claim 12, wherein the logic circuit coupled to said transmission means requires both an inverted signal and a non-inverted signal and the inputs of the logic circuit for the inverted and non-inverted signals are connected to the inverted and non-inverted signal outputs of said decoupling means.
 14. An electronic circuit as claimed in claim 1, wherein the input of said transmission means comprises a first input and a second input, the output of said transmission means comprises a first output and a second output, and said transmission means comprises a first transmission means coupled to said first input and a second transmission means coupled to said second input, and said decoupling means is connected to at least one of said first transmission means and said second transmission means.
 15. An electronic circuit as claimed in claim 14, wherein said logic circuit is coupled to said first and said second transmission means.
 16. An electronic circuit as claimed in claim 14, wherein said decoupling means is adapted to decouple said first and said second transmission means from the input capacitance of said logic circuit.
 17. An electronic circuit as claimed in claim 14, further comprising regenerating means for regenerating the signal between at least one of said first and second transmission means and said logic circuit.
 18. An electronic circuit as claimed in claim 14, comprising means for transmitting a signal to the input of said first transmission means and means for transmitting an inversion of said signal to the input of said second transmission means, and wherein said decoupling means comprises a first output for outputting an inverted signal to said logic circuit and a second output for outputting a non-inverted signal to said logic circuit.
 19. An electronic circuit as claimed in claim 14, further comprising means for outputting a regenerated signal from said first transmission means between said first transmission means and said output.
 20. An electronic circuit as claimed in claim 19, wherein said signal regenerating means comprises a first and second field effect transistors each having a gate, a source and a drain, the drain of the first transistor being connected to the source of the second transistor and to said first output, said first transmission means being connected to the gate of said first transistor and means for applying a potential difference between the source of said first transistor and the drain of said second transistor.
 21. An electronic circuit as claimed in claim 1, wherein said logic circuit coupled to said transmission means comprises a summing circuit, for example an XOR gate.
 22. An electronic circuit as claimed in claim 1, wherein said transmission means comprises a carry propagate circuit for determining a carry-out signal from a carry-in signal.
 23. An electronic circuit as claimed in claim 1, wherein said electronic circuit is incorporated in a full adder circuit.
 24. An electronic circuit as claimed in claim 1, further comprising a plurality of electronic circuits as claimed in claim 1, wherein the output of a circuit is coupled to the input of an adjacent circuit.
 25. An electronic circuit comprising first and second signal generating means, each having an input and an output, circuit means between the output of said first signal generating means and the input of said second signal generating means, and a logic circuit coupled to the input of said first signal generating means.
 26. An electronic circuit as claimed in claim 25, wherein at least one of both said first and second signal generating means comprises an inverter.
 27. An electronic circuit as claimed in claim 25, wherein said circuit means is adapted to operate on the signal output from said first generating means.
 28. An electronic circuit as claimed in claim 27, wherein said circuit means is adapted to process a carry-in signal for determining a carry-out signal.
 29. An electronic circuit as claimed in claim 25, wherein said logic circuit comprises a summing circuit.
 30. An electronic circuit as claimed in claim 25, including input capacitance reducing means for reducing the input capacitance of said circuit coupled to the input of said first signal generating means on the input of said first signal generating means.
 31. An electronic circuit as claimed in claim 25, wherein said electronic circuit is incorporated in an adder circuit.
 32. An electronic circuit as claimed in claim 25, further comprising a plurality of electronic circuits as claimed in claim 25, wherein the output of the second generating means of a circuit is connected to the input of the first signal generating means of an adjacent said electronic circuit.
 33. An electronic circuit comprising first and second inputs and first and second outputs, first transmission means between said first input and said first output and second transmission means between said second input and said second output, a circuit having first and second inputs, wherein the first input is coupled to the first transmission means and the second input is coupled to the second transmission means.
 34. An electronic circuit as claimed in claim 33, wherein said first input of said circuit is coupled to the first input of said first transmission means and the second input of said circuit is coupled to the second input of said second transmission means.
 35. An electronic circuit as claimed in claim 33, wherein said first transmission means comprises circuit means for operating on the signal received at the input of said first transmission means.
 36. An electronic circuit as claimed in claim 35, wherein said circuit means comprises a carry propagate circuit adapted for determining a carry-out signal from a carry-in signal at the. input of said first transmission means.
 37. An electronic circuit as claimed in claim 33, wherein said second transmission means comprises circuit means for operating on the signal received at the input of said second transmission means.
 38. An electronic circuit as claimed in claim 37, wherein said circuit means comprises a carry propagate circuit for determining a carry-out signal from a carry-in signal at the input of said second transmission means.
 39. An electronic circuit as claimed in claim 33, wherein the input of said second transmission means is adapted to receive an inverted version of the signal received at the input of said first transmission means.
 40. An electronic circuit as claimed in claim 33, further comprising decoupling means coupled between the input of said first transmission means and the input of said circuit for decoupling an input capacitance of said circuit from the input of said first transmission means.
 41. An electronic circuit as claimed in claim 33, further comprising decoupling means coupled between the input of said second transmission means and an input of said circuit for decoupling an input capacitance of said circuit from the input of said second transmission means.
 42. An electronic circuit as claimed in claim 40, wherein said decoupling means comprises an NMOS FET.
 43. An electronic circuit as claimed in claim 42, wherein said decoupling means further comprises a PMOS FET in which one of the drain and the source of the PMOS FET is coupled to one of the source and the drain of the NMOS FET.
 44. An electronic circuit as claimed in claim 33, further comprising an inverter at the output of said first transmission means.
 45. An electronic circuit as claimed in claim 33, further comprising an inverter at the output of said second transmission means.
 46. An electronic circuit as claimed in claim 44, wherein said inverter comprises an N-type transistor and a P-type transistor and said transmission means is coupled to the control input of said N-type transistor.
 47. An electronic circuit as claimed in claim 44, wherein said inverter comprises a differential buffer.
 48. An electronic circuit comprising first and second transmission means having a respective input and a respective output and a differential buffer having a first input coupled to the output of said first transmission means and a second input coupled to the output of said second transmission means. 